Flexible ultrasound-induced retinal stimulating piezo-arrays for biomimetic visual prostheses

Electronic visual prostheses, or biomimetic eyes, have shown the feasibility of restoring functional vision in the blind through electrical pulses to initiate neural responses artificially. However, existing visual prostheses predominantly use wired connections or electromagnetic waves for powering and data telemetry, which raises safety concerns or couples inefficiently to miniaturized implant units. Here, we present a flexible ultrasound-induced retinal stimulating piezo-array that can offer an alternative wireless artificial retinal prosthesis approach for evoking visual percepts in blind individuals. The device integrates a two-dimensional piezo-array with 32-pixel stimulating electrodes in a flexible printed circuit board. Each piezo-element can be ultrasonically and individually activated, thus, spatially reconfigurable electronic patterns can be dynamically applied via programmable ultrasound beamlines. As a proof of concept, we demonstrate the ultrasound-induced pattern reconstruction in ex vivo murine retinal tissue, showing the potential of this approach to restore functional, life-enhancing vision in people living with blindness.


Supplementary Notes Supplementary Notes 1: Fundamentals of electrical retinal stimulation.
Electrical stimulation of the nervous system has been studied for a long time. Electrode-based extracellular stimulation generally operates by injecting current into the tissue of interest by placing a single electrode or a group of electrodes nearby. For example, in electronic retina implants 1 , the electrode array is placed close to the retina to form an electrochemical interface with saline. The current injected by the stimulation electrode passes through the retinal tissue to reach the return electrode. The electric current delivered to the extracellular area causes the charge on the cell membrane of the retinal neurons to redistribute. The action potential is initiated when the membrane depolarization exceeds the threshold.
The effects of stimulus pulse proximity between the electrode and neuron can be theoretically predicted using models of electrical stimulation. In a Rattay model 2 , the extracellular electrode serves as an ideal point source, with a fixed distance from a uniform, infinitely long axon, which is useful to form a basic understanding of neural activation via extracellular electrodes. The polarity and extent of membrane polarization will change in response to the axonal stimulation with a monopolar electrode. In cathodic stimulation, negative charges accumulate on the outside of the membrane under the electrode, driving the intracellular movement of the positive charges from the adjacent compartments to this area, yielding strong membrane depolarization near the electrode and weak hyperpolarization away from the electrode. However, in anodic stimulation, strong hyperpolarization occurs in the membrane segment close to the electrode, and weak depolarization occurs at the distal end due to the reverse electric field.
For the electrical stimulation of the retina, a complex neural network, charge redistribution on the membrane of soma, axons, and dendrites will all contribute to the depolarization of the retinal neurons. The initial segment of the axon of retinal ganglion cells is located at the proximal end to the soma and contains high-density sodium channels. Extracellular stimulation of the retinal ganglion cells experimentally demonstrates that the axon initial segment possesses the smallest activation threshold, followed by other axonal sections and the soma, with the dendrites showing the maximum threshold 3 . At the subcellular level, an action potential initiated on one neuronal component may propagate to another neuronal component, thereby significantly affecting the time and space response dynamics of the cell 1 .

Supplementary Notes 2: Acoustic impedance matching in ultrasonic transmission.
Acoustic interface that refers to the boundary between two materials with different acoustic impedances directly affects the transmission of ultrasound 4 . A certain amount of ultrasound energy is transmitted across the interface, and a certain amount of energy is reflected when acoustic waves enter an acoustic interface with normal incidence. For an ultrasound receiver, the reflection of ultrasound off the surface is inevitable because of the acoustic impedance mismatch between the ultrasound media (e.g., ultrasound gel, tissue) and the device. The power reflected can be written wherein d and m are the acoustic impedance of the device and media, respectively. For example, the acoustic impedances for piezoelectric ceramics and crystals are generally 20-30 MRayls, for 1-3 composite are generally 10-15 Mrayls, but the acoustic impedance of the ultrasound gel or tissue is relatively low (~ 1.5 MRayls). A useful method to reduce the reflection is to add an acoustic matching layer between the piezoelectric layer and ultrasound media. Ideal acoustic impedances of a matching layer ( ml ) can be evaluated as 6 ultrasound has shorter wavelengths for higher resolution. Our receiving piezoelectric array is with 32 elements and each element size is 1 × 1 mm 2 . The gap between the two elements is 0.5 mm. To enhance the resolution of the acoustic beam without affecting the operation of the adjacent piezoelectric element, the wavelength of ultrasound should be less than 0.5 mm. For example, the wavelength 3.3-MHz ultrasound is ~ 0.47 mm, is possible to meet the resolution requirements.
Third, the attenuation of ultrasound energy is linearly dependent on the ultrasound frequency.
High frequency will cause high attenuation, thereby reducing transmission efficiency.
Therefore, combining the above aspects (the usual range of medical ultrasound, wavelength, and attenuation), 3.3-3.5 MHz ultrasound that combines high resolution with low attenuation but in the usual medical range was selected in this work.

Supplementary Notes 4: Acoustic field simulation in Field II.
The acoustic field of the ultrasound transducer was simulated by Field II, which is a program based on the calculation of spatial impulse responses as suggested by Tupholme 9 and Stepanishen 10 .
The concept is similar to the impulse response in any linear system and assumes a homogeneous and bounded medium where the pressure is small enough to ensure linear wave propagation.
Therefore, single-element and multi-element arrays can be processed because the response is simply a superposition of the responses from different elements that are correctly phase aligned.
This approach can be illustrated using Huygens' principle, where the impulse response is computed from the sum of all the spherical surface waves from the aperture region S as 11 wherein |r ⃗1-r ⃗2| is the distance from the transducer at position r ⃗2 to the field point at ⃗1, δ(t) is the Dirac delta function, and c is the acoustic velocity.
In addition, it is notable that the impulse responses are dependent on the relative position between the transmitter and the receiver, hence the term spatial impulse response. It can be used to calculate any type of linear ultrasound field. The emitted acoustic pressure field p(r ⃗1,t) is written where ρ0 is the density of the medium and ∂v(t)/∂t is the acceleration of the front face of the transducer.

Supplementary Notes 5: Resolution calculation of the focused single transducer.
The spherical focusing confirmation of the transducer can not only enhance the magnitude of the acoustic excitation but also effectively improve the lateral resolution of the ultrasound beam.
The lateral resolution (Rlateral) is mainly dependent on device geometry, which can be theoretically estimated by 12 The f-number is used to describe the ratio between the focal length and the aperture size. In our

Supplementary Tables
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Supplementary Figures
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